Look, in my opinion accepting BTC exposes a business to unnecessary forex exposure which in general would be avoided.  The ONLY solution to this problem is a transparent forward/futures market which at a marginal cost will allow businesses to easily offload their exposure.  Even this is a hack and so unless the benefits (tangible benefits) outweigh this issue,  bitcoins are going to have a tricky time becoming mainstream.

There is a problem with thinking that people will drop out of mining - once the barrier to entry has been passed (purchasing a rig) the only direct cost a miner faces is the cost of electricity and I suppose there needs to be a marginal reward for effort.  If difficulty goes through the roof it will still make sense to continue mining to make the most of a sunk cost (the rig).  I suspect the network computational power will be sticky down - admittedly it may put a damper on growth in the power, but I doubt people will drop out.  Everyone will just have to make he most of the equipment they have bought.

The one saving grace is that in the long run the mechanics are designed to allow for "fair" compensation for the risk of mining - but we talking an ROI of around 10 - 15% p.a. not what people expect at the moment.